Hardenable Carbon Steels

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Introduction to Carbon Steels

Carbon steels are produced in greater tonnage and have wider use than any other metal because of their versatility and low cost. There were several compelling reasons why carbon steels proved satisfactory when industries reevaluated their material choices:

Their hardenability, though less than that of alloy steels, was adequate for many parts. For some applications, shallower hardening actually presented an advantage by minimizing quench cracking.

Advancements in heat treating methods, such as induction hardening, flame hardening, and "shell quenching," made it possible to obtain higher properties from carbon steels than previously achievable.

The development of new compositions expanded the carbon steel group, allowing for more discriminating selection based on specific application requirements.

Today, there are almost 50 grades available in the nonresulfurized series 1000 carbon steels and nearly 30 grades in the resulfurized series 1100 and 1200. The versatility of carbon steels has been further extended by the availability of various grades with lead additions, which enhance machinability while maintaining essential mechanical properties.

Classification of Hardenable Carbon Steels

Carbon steels can be divided into three primary classifications based on carbon content, each with distinctive properties and applications.

Low Carbon Steels (0.10 to 0.25% C)

These steels typically undergo three principal types of heat treatment:

• Conditioning treatments, such as process annealing, that prepare the steel for specific fabricating operations
• Case hardening treatments to enhance surface hardness
• Quenching and tempering to improve mechanical properties

The improvement in mechanical properties gained by straight quenching and tempering of low-carbon steels is usually not cost-effective without additional processing.

Process annealing is commonly applied to low-carbon cold-headed bolts made from cold-drawn wire. The strains introduced by cold working can weaken the heads, making them susceptible to breakage through severely worked portions under slight additional strain. Process annealing mitigates this condition, though temperatures used are close to the lower transformation temperature, resulting in considerable reduction of normal mechanical properties in the shank.

A more suitable alternative is stress relieving at approximately 1000°F (540°C). This treatment retains much of the strength acquired during cold working while providing adequate toughness. A common practice combines stress-relieving with a quench from the upper transformation temperature or slightly above, producing mechanical properties approaching those of cold-drawn stock. Water solutions of soluble oil are frequently used as quenching media, producing two desirable results:

• The parts' surface acquires an aesthetically pleasing black color accepted as a commercial finish
• The quenching speed is moderated to prevent fully quenched hardness, eliminating the need for tempering

Heat treatments can also significantly improve machinability. The generally poor machinability of low-carbon steels (except those containing sulfur or other special alloying elements) results primarily from the high proportion of free ferrite to carbide. While this fundamental characteristic cannot be changed, machinability can be enhanced by converting the carbide into its most voluminous form, pearlite, and dispersing it evenly throughout the ferrite matrix. Normalizing is commonly used, but optimal results are achieved by quenching the steel in oil from 1500-1600°F (815-870°C). With the exception of steels 1024 and 1025, this treatment forms no martensite, eliminating the need for tempering.

Medium Carbon Steels (0.25 to 0.55% C)

Due to their higher carbon content, these steels are typically used in the hardened and tempered condition. By selecting appropriate quenching media and tempering temperatures, a wide range of mechanical properties can be achieved. These steels are the most versatile of the three carbon steel groups and are commonly used for crankshafts, couplings, tie rods, and numerous other machinery parts requiring hardness values between 229 and 447 HB.

This group shows a continuous transition from water-hardening to oil-hardening types. Their hardenability is highly sensitive to chemical composition changes—particularly manganese, silicon, and residual elements—as well as grain size. These steels also demonstrate sensitivity to section changes during heat treatment.

The heating rate for quenching significantly affects hardenability under certain conditions. With non-uniform structures resulting from severe banding or improper normalizing/annealing, extremely rapid heating (as in liquid baths) may not allow sufficient time for carbon and other elements to diffuse in the austenite. This can result in non-uniform or low hardness unless heating duration is extended. When heating steels containing free carbide (such as spheroidized material), sufficient time must be allowed for carbide dissolution; otherwise, the austenite will have a lower carbon content than the steel's chemical composition indicates, potentially yielding disappointing results.

Medium-carbon steels typically require normalizing or annealing before hardening to achieve optimal mechanical properties after subsequent hardening and tempering. While parts made from bar stock often receive no treatment prior to hardening, forgings commonly undergo normalizing or annealing. Most bar stocks (both hot-finished and cold-finished) are machined as received, except for higher-carbon grades and smaller sizes, which require annealing to reduce as-received hardness. Forgings usually undergo normalizing to avoid extreme softening and reduced machinability that result from annealing.

"Cycle treatment" is sometimes employed, where parts are heated as for normalizing and then cooled rapidly in the furnace to a temperature above the S-curve nose—within the transformation range that produces pearlite. The parts are then held at temperature or cooled slowly until the desired transformation occurs, after which they are cooled as convenient. This process typically requires specially arranged furnaces, with treatment details varying based on available equipment.

Cold-headed products are commonly manufactured from these steels, especially those containing less than 0.40% C. Process treating before cold working is usually necessary because higher carbon content reduces workability. For certain applications, these steels may be normalized or annealed above the upper transformation temperature, but spheroidizing treatments are more common. The required degree of spheroidization depends on the specific application. After shaping operations, parts typically undergo quenching and tempering.

These medium-carbon steels find widespread use in machinery parts for moderate-duty applications. When parts require machining after heat treatment, maximum hardness is typically limited to 321 HB, and is frequently much lower.

Salt solutions serve as effective quenching media. While not hazardous to operators, their corrosive action on iron or steel equipment components can be severe. For light sections or applications with moderate property requirements, oil quenching often presents a viable alternative that virtually eliminates breakage problems and effectively reduces distortion.

Austenitizing temperatures vary widely to meet specific requirements. Lower temperatures are preferred for higher-manganese steels, light sections, coarse-grained material, and water quenching. Conversely, higher temperatures are necessary for lower manganese content, heavy sections, fine grain structures, and oil quenching.

These steels are commonly used to manufacture hand tools such as pliers, open-end wrenches, screwdrivers, and certain edged tools like tin snips and brush knives. Cutting tools typically undergo local quenching on cutting edges using water, brine, or caustic solutions, followed by appropriate tempering. In some cases, the edge undergoes time quenching, while the remainder receives oil quenching for partial strengthening. Hand tools like pliers, wrenches, and screwdrivers made from these steels typically undergo complete or local water quenching followed by appropriate tempering.

High Carbon Steels (0.55 to 1.00% C)

Carbon steels with higher carbon contents have more limited applications than medium-carbon steels due to increased fabrication costs resulting from decreased machinability, poor formability, and reduced weldability. They also exhibit greater brittleness in the heat-treated condition.

Higher-carbon steels such as 1070 to 1095 are particularly suitable for springs requiring resistance to fatigue and permanent deformation. They also excel in nearly fully hardened conditions (Rockwell C 55 and higher) for applications where abrasion resistance is paramount, such as agricultural tillage tools like plowshares and knives for cutting hay or grain.

Forged parts should undergo annealing to refine the forging structure—critical for producing high-quality hardened products—and to reduce hardness for cold trimming of flash and economical machining. Standard annealing practices followed by furnace cooling to 1100°F (590°C) are suitable for most applications.

Most parts manufactured from these steels undergo conventional quenching, though special techniques may be necessary in certain cases. Both oil and water quenching find application—water for heavy sections of lower-carbon steels and cutting edges, and oil for general use. Austempering and martempering often yield excellent results, offering three primary advantages: significantly reduced distortion, elimination of breakage in many cases, and greater toughness at high hardness.

For heavy machinery components such as shafts and collars, steels 1055 and 1061 may be used—either normalized and tempered for lower strength applications or quenched and tempered for moderate strength requirements. While other steels in this category may be suitable, the specific carbon and manganese combination in these two makes them particularly well-adapted for such applications.

It's essential to remember that even with favorable hardenability factors and drastic quenching, these steels are fundamentally shallow-hardening compared to alloy steels. Carbon alone, or combined with manganese in the amounts present in these steels, does not significantly promote deep hardening. Consequently, suitable section sizes are definitely limited. Despite this limitation, breakage risk remains significant and requires careful management during treatment, especially with section changes.

Hand tools manufactured from these steels include open-end wrenches, Stillson wrenches, hammers, mauls, pliers, screwdrivers, and cutting tools such as hatchets, axes, mower knives, and band knives. The carbon and manganese combinations used may vary considerably for identical tool types, depending partly on available manufacturing equipment and partly on experience with or preference for specific compositions. Some tools may incorporate manganese content lower than standard when it facilitates handling of a particular carbon range, though for many applications, a combination of lower carbon and higher manganese would perform equally well.